<p> This work incorporates information from two sources in order to examine the
nature of natural selection acting on phenotypic characters in Drosophila melanogaster
along a North South cline. Isofemale lines were established from flies collected along a
North South cline extending from Winnipeg, Manitoba to Tampa Bay, Florida. Offspring
from different lines within each position were then cultured under standardized conditions
and used to examine phenotypic variation in 10 morphological characters along the cline.
In a separate set of experiments, flies from Vineland, Ontario were mated in a half-sib
design in order to estimate the genetic covariance of the set of 10 characters. The results
from the clinal and heritability experiments were then combined using Lande's (1979)
equation,&.= Gp-ls, to estimate the net selective differentials and net selection gradients
for each adjacent set of populations. The study concluded that:
1) Clinal variation is non-linear, with larger flies in the
middle latitudes and smaller flies in the north and south.
2) Selection appears to act primarily on body characters in
the north (wing width and femur length) and head characters
in the south (eye and face width).
3) Scutellum width and wing length generally moderate the
prevalent trends in directional selection on the other
characters through antagonistic correlated responses.
4) Clinal patterns of variation may not be at equilibrium, but
instead dominated by seasonal responses to selection
pressures. </p>